This disclosure relates generally to heliostats having reflectors configured to redirect sun light to a target or receiver, and in particular to heliostat drive assemblies configured to dynamically control the position of the reflectors.
In Concentrating Solar Power (CSP) plants, arrangements of heliostats reflect sunlight toward a receiver mounted atop a tower containing a working fluid. One type of receiver transfers incident radiant energy to the working fluid to produce high-pressure, high-temperature steam through the means of a heat exchanger or a phase change of the working fluid itself. The working fluid can be water, air, or a salt material heated to a molten state. The output steam can facilitate a variety of applications, such as electrical power generation, enhanced oil recovery, and desalination. Heliostats are generally mounted on the ground in an area facing or surrounding the receiver tower. Each heliostat has a reflector: a rigid reflective surface, such as a mirror, that tracks the sun through the actuation of a heliostat drive mechanism about at least one axis. Sun-tracking involves orienting the reflector throughout the day so as to optimally redirect sunlight from the sun toward the receiver and maintain the desired temperature of the working fluid.
One approach to constructing a heliostat field is to utilize a small amount of comparatively large heliostats (e.g., greater than between 50 and 150 m2). In such a power plant, having a fewer number of heliostats may necessitate the manufacture of very precise, and thus very expensive, components for the positioning of the reflective surfaces. Another approach, however, is to use a large amount of comparatively small heliostats (e.g., between 1 and 10 m2), such as with reflective surfaces that measure between 1 and 3 m on each side. Such an approach may be more efficient at redirecting sun light because there are more individually adjustable reflective surfaces. In addition, smaller heliostats may be cheaper to produce and easier to assemble, decreasing installation time and operations costs. However, a plant comprising more heliostats will necessarily require the same amount of additional drive assemblies, increasing the number of repeated steps during installation. Accordingly, there is a need for heliostat assemblies that are both economical to manufacture and efficient to install.
One problem with controlling the positioning of heliostats is that sun-tracking must be precise, and the orientation of the reflective surface must be under a certain prescribed angular tolerance at all times. This is because accurate positioning of the reflectors is necessary to maintain efficiency of the power plant. However, wind and other environmental factors may apply loads to the reflector that move the reflector away from its preferred orientation at a given point in time of tracking the sun. Such forces may contribute to motion in the drive mechanisms and may cause deviation in the reflector from its preferred orientation at a given instant. Manufacturing tolerances between the components of the heliostat may also contribute to backlash, undesirable movement and non-linearity in the drive systems. These tolerances may undesirably result in a greater amount of variation between the calculated and the actual reflector orientation. Consequently, such variation may lead to inefficient tracking of the sun and decreased efficiency in reflecting radiant solar energy toward the receiver, as well as excessive wear and tear to mechanical parts.